In the processing of wood to produce paper the wood is reduced to individual fibers. The fibers are produced either by mechanically abrading wood or by chemically processing wood chips to remove the lignin which binds the fibers together in raw wood. The individual fibers are then typically subjected to further mechanical and chemical processing to improve the properties of the fibers and thus produce a better grade of paper.
Recycled fibers are typically freed from recycled paper for fiber board by repulping the recycled material into individual fibers. Again the recycled fiber are subjected to mechanical and chemical processing to clean and improve the properties of the recycled fibers.
Bleaching of the fibers is one very important chemical process to which raw and recycled wood fibers are subjected to lighten and brighten the fibers in order to produce a lighter, more valuable grade of paper. The bleaching of paper pulp today is a complicated, many step process, and can employ numerous bleaching agents, including chlorine, oxygen, ozone, chlorine dioxide and hydrogen peroxide. Many of the bleaching chemicals must be evenly dispersed throughout the mass of fibers, or an undesirable mottled, or uneven, bleaching of the fibers results.
For many of the processes involving wood fibers in the papermaking process, the fibers are handled as so called "medium consistency" stock, consisting of about twelve percent fibers by weight dispersed in eighty-eight percent water by weight. Medium consistency stock contains about the maximum fiber content at which the stock can still be handled as a liquid by pumping. However, medium consistency stock is more like a solid than a liquid, easily holding it's shape if piled on a flat surface. Under normal conditions, medium consistency stock is difficult to mix with any additive.
However, if the medium consistency stock is subjected to high levels of shear, it becomes very fluid, approaching the fluidity of ordinary water, and under the turbulent conditions produced by the high shear levels rapidly mixes. Producing the necessary level of shear in the medium consistency stock requires between five and fifty megawatts per cubic meter, or up to fifty kilowatts per liter of stock. This amount of energy would bring the stock to a boil in about six seconds. In order to keep energy costs to a reasonable level, the volume of fluid flow subjected to high shear must be kept to a minimum. Minimizing the volume subjected to high shear results in the fluids being subjected to the shear for only a small fraction of a second. The problem with using a very small volume for mixing is that the distribution of chemicals and pulp must be uniformly distributed on a scale consistent with the mixing volume which is subjected to high shear.
A number of known devices exist for mixing medium consistency paper pulp stock. For example, U.S. Pat. No. 4,435,085 to Luthi et al., which utilizes a disk-shaped rotor rotating between fixed disks. Another device, U.S. Pat. No. 5,378,321 to Delcourt, discloses mixing between conical rotating surfaces which produce high shear. Yet a further device, disclosed in U.S. Pat. No. 5,466,334 to Fredriksson et al., has a toothed roll rotating within a housing and opposed to a toothed plate. These existing devices mix medium consistency pulp stock by providing regions of high shear where mixing can take place.
However, if improved distribution of the chemicals to be mixed with the pulp could be achieved, a smaller region of high shear could be used with the result that less power would be required for mixing and/or greater uniformity of chemical distribution within the pulp could be achieved.